nexus-maven-plugin: discover nexusURL, serverAuthId, user, password wherever possible

in all cases EXCEPT staging mojos:

nexusUrl: Check mirrors, repository definitions to find a Nexus instance. Allow override via property defined in settings.xml or CLI.

in staging cases:

nexusUrl: Check distributionManagement, and allow override via settings.xml/CLI. If undefined in these, check mirrors and repos, and use these to SUGGEST a nexusUrl value. If this is accepted, use that url as a basis for serverAuthId/user/password.

in BOTH cases:

serverAuthId/user/password: try to use id of distMgmt repository. if that repo isn't used, try to match the nexusUrl with one of mirrors/repos defined, and use the id from that. failing that, prompt.